After the discovery of infrared radiation by William Herschel in
1800 and the discovery of the thermoelectric effect by Thomas Johann
Seebeck in 1821, scientists have tried to measure this long wave
radiation with the help of thermocouples and thermophiles. The military
has been particularly interested in this invisible, warm radiation, and
after World War II the armed forces invested significant amounts of
money into research in this field. Consequently, great progress in the
development of infrared detectors took place in the last third of the
20th century and elegant cameras to measure radiation and hence
temperature were built. With the help of new systems the spectrum of
civil applications grew as well: from construction thermography to its
use in electrotechnology, environment engineering, civil engineering and
medicine. It can be applied everywhere where processes inside of an
object lead to the change of temperature of its surface. Bearing in mind
the current discussion on climatic changes, construction thermography
gains importance, because it quickly discovers heat losses of building
envelopes. It is in compliance with the Thermal Insulation Ordinance.
Thermography is the determination of surface temperatures of objects and
bodies with the help of infrared photography [1-3]. A special-purpose
camera captures what the human eye cannot see [4-6]. The camera consists
of an infrared-permeable lens, a transmission line and a sensitive
detector. The detector converts radiation into electric signals. After
processing they are transformed into pixels so that the thermogram
appears on the screen.

2. THEORETICAL BASIS

The theory of infrared thermography is based on three well-known
radiation laws [2,6]:

--the law about the relation between emission and absorption found
by Kirchhoff,

--Planck's law of radiation,

--the Stefan-Boltzmann law.

Kirchhoff 's law means that a body that absorbs much also
emits much. When a "grey" body stands opposite a
"black" body, absorption and emission are equal in the thermal
balance. Thermography is based on the emission of objects. Therefore it
uses the emission coefficient [epsilon] as the ratio of emissivity E of
a real body to the emissivity [E.sub.s] of the black body under the
same temperature

[epsilon] = E/[E.sub.s]. (1)

Thus the emission coefficient [epsilon] is nondimensional; it lies
between 0 and 1 and depends on the wavelength, on the temperature and
the surface texture of the body. The Planck radiation law describes the
specific spectral radiation M emanating from the idealized black body:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where [lambda] is wavelength, T is the absolute temperature, h is
the Planck constant and c is the speed of light. If the specific
spectral radiation M is plotted over the wavelength [lambda] as a
function of temperature, typical Planck curves will result. In them it
can be seen that the maximum of the curves shifts with growing
temperature towards a smaller wavelength according to the Wien
displacement

[[lambda].sub.max] = 2898/T. (3)

Stefan and Boltzmann looked at the emission of a surface over all
wavelengths, integrated the Planck law and discovered that the radiant
power I in W [m.sup.-2] grows with the fourth power of the temperature:

I = [sigma][T.sup.4], (4)

where [sigma] = 5.67 x [10.sup.-8], W [m.sup.-2] [K.sup.-4].

In the thermographic camera the detector determines the traceable
range of wavelengths. It is important to stay within the area of
atmospheric windows. Only in these windows is the atmosphere's
transmissivity high enough for infrared radiation. That is why most
cameras operate in the area of 3 to 5 or 8 to 12 [micro]m. Another
window exists in the near infrared between 0.78 and 3.0 [micro]m.

The emission coefficient is generally determined experimentally
[3,4]. There are two possibilities for that. According to the effect of
Seebeck, a thermocouple can be used to measure the temperature of the
surface that is to be examined. Then the emission coefficient in the
camera is changed until the camera shows the same temperature as the
thermocouple does. The second method uses a commercial emission sticker.
Its emission coefficient [epsilon] is known. The thin paste-on label is
stuck to the surface. After the infrared exposure the temperature of the
sticker is compared to the surface next to it in the thermogram.

An even easier way is the use of a calibrated black spray. In the
meantime many extensive tables and charts with emission coefficients
have become available [2-4]. But one has to consider that not every
surface texture can be covered with a spray. As for metals, their
different oxidation coverage may exclude application of sprays.

3. CONSTRUCTION THERMOGRAPHY

The main application of infrared thermography in the civil sector
lies in construction thermography. Fortunately, most materials used in
the building industry have emission coefficients between 0.90 and 0.96
(Table 1) [1,4,6]. Therefore good assessment of thermal properties of a
building can be made with only one exposure with the same [epsilon].

Post-processing of the thermograms for zinc or copper clad
components can then still be done with the computer.

A basic condition for using thermography on buildings is a
difference of 20 K between the inside and the outside temperatures. In
the literature 10 K are sometimes considered enough. It means that
examinations of buildings are reasonable only during the winter when the
surrounding temperature lies around the zero-point. Inside exposures are
of greater significance because atmospheric conditions such as wind,
rain, snow or sun as well as conditions of the building itself like
ventilated facades have an effect on the results of the exposure. While
heat bridges can be seen from outside, there are cold bridges observable
inside. Cold areas clearly stand out on walls, loft conversions, corners
on the floor or window frames. But heated floors and heaters on walls
can also be made visible. Their position and length can be exactly
determined. Furthermore, it is possible to find leaking or plugged
heating systems, badly done insulation or hidden timbered framework,
which has been plastered over. Pictures, taken with thermographic
cameras, are admitted in court because they provide unambiguous proof of
botched construction works. Construction firms benefit from them as well
since they can photograph critical areas in order to design targeted
constructional measures before reconstructing old buildings. Infrared
exposures can be filed together with regular photographs of the same
object and tables with thermographic results for later analysis.

4. SELECTED EXAMPLES

As examples, town halls of the four big Hanseatic cities of
Rostock, Stralsund, Wismar and Greifswald were photographed. They all
lie on the coast of the Baltic Sea of Mecklenburg-Western Pomerania.
Together with every thermographic exposure a regular visual photograph
was taken. In addition, data about the surrounding temperature,
humidity, distance of the exposure from the object as well as of the
emission coefficient were collected. The coefficient was tuned in
consistently as 0.95. All city halls had been reconstructed and
renovated. The aim of the thermographic pictures was to find out whether
there are "climate sinners" among municipal administration
buildings. During the reconstruction it had been very difficult to
rebuild the premises according to thermal insulation standards as well
as to the preservation requirements of historic monuments. This is not
always easy, but it was mostly successful thanks to the thick outer
walls that were commonly used in former times for reasons of
construction engineering rather than because of thermal insulation. Some
window installations were to be reduced [5].

Built in the 13th century, the town hall of Rostock (Fig. 1) has
had a baroque front since the middle of the 18th century. In 1995 it was
completely reconstructed and renovated. The old, thick walls insulate
the building excellently. The Thermal Insulation Ordinance [7] was not
observed with the windows on the upper floors. The town hall of
Stralsund (Fig. 2) received a gothic facade in 1340. Reconstruction was
finished in 2004. In the upper part of the gable there are no windows,
but apertures that prevent the facade from collapsing under stormy
weather conditions.

The town hall of Wismar (Fig. 3) was constructed between 1817 and
1819 in the classical style. After a fire in the roof it was reopened in
1992. Single red-white windows show that the rooms behind them are
heated more than the remaining rooms.

In comparison to the others, the town hall of Greifswald (Fig. 4)
is the worst insulated. After the reconstruction of 2000, many heat
bridges remained. The building, dating from late gothic times, was
erected around in 1400. After two fires in the 18th century it was
rebuilt as a baroque edifice.

A different example is localization of cold spots inside a new
detached house (Fig. 5). The infrared photograph provides evidence of
the deficient insulation and shows that heat bridges lead to
inadmissibly low surface temperature. This causes growth of mold as soon
as the measured temperature falls below the dew point. With a room
temperature of 21 [degrees]C and a high relative air humidity of 70%,
the dew point lies at approximately 15.3 [degrees]C.

An application area of thermography is also detection of the
position of heating tubes in floors and walls (Figs. 6 and 7). When, for
example, a supplementary chimney is to be built in the living room or a
tie is to be placed in an outer wall, it is important to know exact
location of the pipes. In the same way any leakage can be detected in
heating systems because a leak produces a wide heat spot in the
thermogram. Another application of thermography is measurement of
moisture in building materials [8].

5. CONCLUSIONS

Infrared construction thermography is a modern contactless
measurement procedure for diagnostics and analysis of buildings [2,9].
Pre-conditions are scientific expert knowledge concerning physical
fundamentals as well as a minimum of constructional and technical
understanding. With the help of construction thermography, compiling
evidence for heat losses is possible. One can locate heat and water
leakages or make extensive analysis of the actual condition of edifices.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Thermograms and visual photographs are taken from a fixed position
and are analysed side by side in the laboratory. On and in the building
there must be stationary conditions. Outside exposures must be taken
before sunrise, otherwise solar radiation will hit the lens through
reflections. Thus the measurement would be adulterated. Ideal conditions
for the use of infrared thermography are outside temperatures around 0
[degrees]C, no wind or rain and inside temperatures around 20
[degrees]C. Then a temperature resolution up to 0.2 K can be reached.
The temperature measurement error is about 2%.

[FIGURE 6 OMITTED]

Efficiency of the method is illustrated by thermograms of four town
halls and for three particular cases of building inspection.

[FIGURE 7 OMITTED]

ACKNOWLEDGEMENTS

The author wants to thank Dipl.-Ing. (FH) Ernst Langnau
(thermography) and Sabrina Wolf (photography) for their support with the
pictures taken of the town halls.